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    Physics General Subjects Thread

    Sprut-B
    Sprut-B


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    Post  Sprut-B Sat Oct 01, 2022 4:41 pm

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    kvs
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    Post  kvs Wed Jan 25, 2023 12:09 am

    https://news.mit.edu/2023/roman-concrete-durability-lime-casts-0106

    Riddle solved: Why was Roman concrete so durable?

    The ancient Romans were masters of engineering, constructing vast networks of roads, aqueducts, ports, and massive buildings, whose remains have survived for two millennia. Many of these structures were built with concrete: Rome’s famed Pantheon, which has the world’s largest unreinforced concrete dome and was dedicated in A.D. 128, is still intact, and some ancient Roman aqueducts still deliver water to Rome today. Meanwhile, many modern concrete structures have crumbled after a few decades.

    Researchers have spent decades trying to figure out the secret of this ultradurable ancient construction material, particularly in structures that endured especially harsh conditions, such as docks, sewers, and seawalls, or those constructed in seismically active locations.

    Now, a team of investigators from MIT, Harvard University, and laboratories in Italy and Switzerland, has made progress in this field, discovering ancient concrete-manufacturing strategies that incorporated several key self-healing functionalities. The findings are published today in the journal Science Advances, in a paper by MIT professor of civil and environmental engineering Admir Masic, former doctoral student Linda Seymour ’14, PhD ’21, and four others.

    For many years, researchers have assumed that the key to the ancient concrete’s durability was based on one ingredient: pozzolanic material such as volcanic ash from the area of Pozzuoli, on the Bay of Naples. This specific kind of ash was even shipped all across the vast Roman empire to be used in construction, and was described as a key ingredient for concrete in accounts by architects and historians at the time.

    Under closer examination, these ancient samples also contain small, distinctive, millimeter-scale bright white mineral features, which have been long recognized as a ubiquitous component of Roman concretes. These white chunks, often referred to as “lime clasts,” originate from lime, another key component of the ancient concrete mix. “Ever since I first began working with ancient Roman concrete, I’ve always been fascinated by these features,” says Masic. “These are not found in modern concrete formulations, so why are they present in these ancient materials?”

    Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognized self-healing capability. “The idea that the presence of these lime clasts was simply attributed to low quality control always bothered me,” says Masic. “If the Romans put so much effort into making an outstanding construction material, following all of the detailed recipes that had been optimized over the course of many centuries, why would they put so little effort into ensuring the production of a well-mixed final product? There has to be more to this story.”

    Upon further characterization of these lime clasts, using high-resolution multiscale imaging and chemical mapping techniques pioneered in Masic’s research lab, the researchers gained new insights into the potential functionality of these lime clasts.

    Historically, it had been assumed that when lime was incorporated into Roman concrete, it was first combined with water to form a highly reactive paste-like material, in a process known as slaking. But this process alone could not account for the presence of the lime clasts. Masic wondered: “Was it possible that the Romans might have actually directly used lime in its more reactive form, known as quicklime?”

    Studying samples of this ancient concrete, he and his team determined that the white inclusions were, indeed, made out of various forms of calcium carbonate. And spectroscopic examination provided clues that these had been formed at extreme temperatures, as would be expected from the exothermic reaction produced by using quicklime instead of, or in addition to, the slaked lime in the mixture. Hot mixing, the team has now concluded, was actually the key to the super-durable nature.

    Impressive.

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    Begome
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    Post  Begome Wed Jan 25, 2023 8:36 pm

    Hey kvs,

    have you heard of the "Dark Comets hypothesis" by Rob Sheldon? (Comets, Water and Big Bang Nucleosynthesis ... I recommend skipping the Introduction and section III as they're a bit esoteric IMO).

    So basically certain properties of the very early universe in the electro-weak era make it electrically conductive and lead to strong primordial magnetic fields, which then discharge into the universe as they later collapse due to the end of that era when nucleosynthesis begins, thus providing more energy for nucleosynthesis, which can thus proceed to produce elements far past Li, even O, which means water (or ice), which means gigantic ice balls due to condensation and freezing, which gives much earlier galaxies (as confirmed now by JWST) and basically does away with the need for Dark Energy and Dark Matter.
    The "dark matter" would simply be comets, which have been shown to have fairly low albedo and would sit in "halo clouds" around galaxies, just as dark matter is expected to by the mainstream; the reason they behave so differently than "non-comet matter" is because comets effectively are matter with its own rocket engine that starts up whenever it either gets big enough or comes near a heat source and thus as a whole has a lot less viscosity and resists this disk-like accretion seen with other matter.

    The author claims that this could solve multiple problems with the mainstream version of standard cosmology, such as CEMP stars, cold white dwarfs, (too) early galaxy formation, matter-antimatter imbalance, missing dark matter and several others.

    Thoughts?
    kvs
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    Post  kvs Wed Jan 25, 2023 10:33 pm

    I think that the line of investigation of this paper is very interesting and this is the sort of research that astrophysics needs. Not
    groupthink, circle-jerk big ego affirmation.

    But I do not think there is an "either/or" situation here. In my view dark matter is the simple failure to recognize that gravity is nonlinear
    and not Newtonian. That is, the gravitational "constant" G is a function of the mass distribution. The MOND correction is basically
    the same premise since we have a G(r) which then gives a deviation from the 1/r^2 force law.

    The reason for the nonlinearity is that gravitons self-interact. We see this effect in the strong force since gluons self-interact and we
    get asymptotic freedom near the force center but increasing attraction away from it (Yang-Mills potential). Gravity does not have the same
    dramatic amplification of the force away from masses as quarks have with the strong force. But it is enough to give us the galaxy rotation
    curves without appealing to missing mass. This is not to say that there is no mass that we do not see. So a comet halo instead of a dark
    matter halo is still possible but the bottom line is that Newtonian theory is only a weak field approximation.

    It is funny that galaxy dynamics simulations use Newton's law of gravity. They do not use GR equations and do not even use "retarded" potentials
    assuming a speed of light gravity. A weak field approximation of GR should be used for galaxies. But GR is not sufficient since it assumes a
    constant G as the whole theory rests on Newton's law. We simply to not have strong field limit understanding of gravity. Black holes are not
    examples of such understanding since GR does not have the strong field limit gravity as its base. In other words, black holes are mathematical
    constructs from some theory and not reality. GR should be formulated starting from a variable G framework.

    In cosmology, having a variable G gets around the ludicrous, ad hoc "hyperinflation" hack. There are too many such deus ex machina fixes
    in astrophysics and cosmology.

    Anyway, we are too far away from full understanding of reality regardless of the tone put out by "authorities" in various scientific subjects.

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    Begome
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    Post  Begome Wed Jan 25, 2023 11:21 pm

    Thank you for your response! I agree that there are a lot of dogmas in science that quench innovative research and get people fired for questioning them. I also agree that it's not either this or MOND, for example, but I'd like to point out that the paper does comment on some of the points you brought up:

    First, the author also proposes that in his theory this inflaton field, which I think is what you're referring to with hyperinflation, is not necessary, because the discharge from the primordial magnetic field "means that the transition from electroweak to nucleosynthesis era is a first order phase transition [...] mapping the coherence of the field onto the coherence of the matter." and thus "the universe achieves a uniform temperature and density that is reflected in the CMBR, without the need for a global inflaton field. Or more precisely, the global magnetic field provides the coherence that was previously attributed to the global inflaton field (albeit indirectly)." (both quotes are from the paper I linked, section 2.4, which also talks about how this theory fixes the "flatness problem" of the Big Bang).

    As to MOND, he writes "Even the unorthodox modified newtonian dynamics/gravity (MOND) has not worked for all galaxy types, leaving theorists without a viable DM candidate (Joyce et al., 2015)." and also, that as a consequence of the way the "comet matter" interacts with the "non-comet matter", its density will decrease around accumulations of matter of higher acceleration without the need for MOND (see section 2.1).

    I do hope that this idea is looked into more in the future, as I personally really like it (Gen. 1:2  Very Happy  ).

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